Creating a doughnut polygon
Problem: You need to cut a hole out of a polygon
This comes up a lot. Perhaps you’re digitizing a lake, and it has an island in the middle of it. Or there’s an area of lawn surrounding a building. If you’re starting from scratch, I recommend drawing lines and converting them to polygons. Similarly, if you need to make a lot of holes, I recommend converting your polygons into lines, editing the lines, then converting back to polygons. That method is simpler for large scale changes.
But sometimes you just need a quick hole or two right now. For instance, you might be working in the Planning Office for the city of Lansing, Michigan. You might be in charge of maintaining their Existing Land Use Map, shown below.
In the southwest quadrant of the map, there is one region of Institutional land that has a Single Residence in the middle. Let’s pretend that house was just built, and you’ve been asked to update the map.
Here’s how you could do it:
1. Make your Land Use polygon layer partially transparent so you can see what you’re doing underneath.
Howto: Right click on the layer in the Table of Contents to bring up Layer Properties. On the Display tab, enter a number greater than 0 and less than 100 in the Transparent % box. 50% is good.
2. Enable editing and double click on the Institutional polygon to show the sketch vertices. Note: Double clicking brings you into Modify Feature mode, allowing you to move vertices in the existing polygon. A single click is Create Feature mode, which draws a new polygon.
3. Right click on the edge of the sketch, near where you want to create the hole, to Insert Vertex. Then create two more vertices next to it.
4. Pull the center vertex in towards one of the outer corners of your hole.
5. Create as many new vertices as needed to drag out to all of the other corners.
6. Prepare to close the empty space and the filled in space by placing two vertices at each location close to another. You’ll end up with a narrow rectangle connecting the edge of the filled space with the edge of the empty space.
7. The idea is to turn this rectangle into a line by merging the vertices at the top and bottom. To do this, turn on snapping to sketch vertices.
Howto: Select snapping from the Editor drop down menu and check the Edit sketch vertices box.
8. Pull the vertices together.
When you click away, that connecting line will disappear, giving you the two distinct areas you want. Now, it’s a simple matter to draw a new polygon for the Residential area within that hole. Just snap to its four corners.
Make sure that snapping to the editable layer, and not just the sketch, is turned on for this.
Note: If you are just creating a visual map, you can draw the smaller polygon on top of the larger, and then set Symbol Levels to ensure they display correctly. (Symbol levels are available from the Advanced drop down in the Symbology tab of the Layer Properties dialog). This is a quick fix if you’re on a tight deadline. It is not recommend if you’re going to be doing analysis on the layer or distributing it to others.
Another Note: There are other ways to do this, such as using the Cut Polygon Features task (for a hole), or the Clip task (for a polygon inside a polygon). ArcMap’s Help describes both of these methods. I like following the steps I have outlined because I can accomplish everything by clicking and dragging without switching between editing tasks. Within GIS, there are many ways to do most things, and you will develop your own style.